Screen printing is used in both the graphic arts and electronics industries. The basic concept in screen printing is to force a viscous material, an ink or paste, through apertures of a stencil screen. A stencil screen has solid areas which prevent printing and penetrable areas which allow the deposition of a pattern onto a substrate. The penetrable areas in the stencil screen can be mesh of the same or variable size, open areas with tie lines holding the solid areas together, or any combination of the above. Optionally, the stencil screen can have some open areas. For graphic arts applications the viscous material is usually an ink which is deposited on paper, fabric, plastic, etc. For electronic applications, thick film pastes of conductors, dielectrics and resistors can be screen printed onto appropriate electronic circuit substrates such as alumina or ceramic "green tape."
Many conventional stencil screens are made by applying a photosensitive emulsion to metal screen mesh. The photosensitive emulsion is exposed to light and hardened in areas corresponding to the solid areas of the stencil screen. The non-hardened areas are then removed with an appropriate solvent. This results in a screen in which some of the open areas have been filled with hardened resist to define a pattern.
When the above described stencil screens are placed on a substrate for printing, both the solid areas and the solid parts of the penetrable areas, i.e., the crossing wires of the mesh, are in contact with the surface of the substrate. Having the mesh contact the printing surface can cause problems for fine printing on smooth, nonporous, hard surfaces, and particularly for screen printing of thick film materials for electronic applications. When the mesh contacts the printing surface, the thick film material prints in a mesh pattern. Some material can run under the intersecting wire or fiber as the weave pattern keeps the grid off contact except at grid intersections. As the mesh is pulled away from the substrate, the paste partially runs together into a solid line, but there is insufficient material in the intersections of mesh with the solid areas to create a solid straight edge. This results in a deep scallop pattern at the edge of the line reflecting the position where each wire in the mesh pattern intersects the edge of the line. It is possible to improve the flow of the thick film material and pastes by decreasing the viscosity, but this can lead to other problems.
With stencil screens in which the solid areas are formed using photosensitive materials, this difficulty is overcome by impregnating the screen with an extra 0.2 to 0.4 mil (0.00051 to 0.0010 cm) thickness of photosensitive resist on one side. After the resist is imaged and developed the extra thickness remains in the solid areas. This is known as an "emulsion build". In screen printing, the stencil screen is placed on the substrate so that the emulsion build contacts the substrate and prevents the mesh areas from contacting the substrate surface. When the thick film paste is forced through the mesh, the paste flows together to fill the channel formed by the edges of the photoresist emulsion build. Thus solid lines are printed.
Some stencil screens are made completely out of metal materials. This can be accomplished by substractive processes, in which metal is removed from areas intended to be penetrable. Metal stencil screens can also be made by additive plating processes. With either type of process there is no opportunity to form a layer that would function as the emulsion build described above.
There exists a need for metal stencil screens which have a layer which functions as an emulsion build. In addition, there is a need for metal stencil screens which have a metal layer which functions as an emulsion; build.